The question of whether aging - the process that converts fit adults into frailer adults with a progressively increased risk of illness, injury, and death - is under genetic control is ambiguous, and its answer depends on what one means by aging. Natural selection can select for genes that retard aging, but only in species and niches where the value of prolonged survival outweighs its costs. Although the form aging takes can be affected by variations at many genetic loci the number of loci that moderate the pace of synchronized decay may be far smaller. Single gene mutants can extend mouse lifespan by over 50%, and genetic selection for small body size also leads to dramatic life span extension in dogs, suggesting strongly that aging can be affected by genetic variations within a species, but identification of genetic differences that discriminate long-lived from short-lived species will require a combination of genetic and physiological analyses.